To add a new repository to the list, find the appropriate section for the repository's language and add a new entry like this for GitHub-hosted repos:
* [username/repo]
or like this for non-GH repos:
* [username/repo](https://somewhere-else.com/username/repo)
Don't link to files or folder within the tree since that messes up the badges. Users will be able to figure out where the code for the current year is if you use the same repo for multiple years so don't worry about it.
Adding your own repos is perfectly fine. If you use multiple languages, then avoid adding your repo to more than 3 sections.
Repositories not hosted on GitHub are fine, but they won't have a "last commit" badge.